1. Technical Field
The present invention relates to a spinal implant assembly for implantation into the intervertebral space between adjacent vertebral bones to provide stabilization and continued post-operative flexibility, as well as restoring anatomical motion to the vertebrae defining the intervertebral space. More specifically, the present invention relates to an artificial intervertebral disc, sometimes referred to as an intervertebral spacer device, for functioning as a load sharing and bearing device for replacement of a damaged, decayed, or otherwise dysfunctional intervertebral disc.
2. Background Art
The spine consists of multiple flexible levels, each level consisting of a system of joints defined by adjacent vertebral bones. The system of joints includes vertebral discs, which are a two-part structure. The disc consists of a nucleus and an annulus. The system allows motion, while facet joints add proper stabilization to the spinal column. The disc allows motion and cushioning at each of the intervertebral joints.
The vertebral joint is subjected to varying loads and problems over time, including disc degeneration due to a variety of reasons. Disc degeneration can be attributed to aging, damage due to excessive loading, trauma, and other anatomical issues. Facet joints of the structure can be compromised due to the same reasons, as well as due to arthritic changes. Severe joint degeneration and failure often cause sufficient pain to require surgical intervention.
The conventional method of treatment for severe pain caused by spine joint problems is fusion at the damaged level of the spine. The treatment, if successful, fuses the damaged section into a single massive bone. The fusion of the joint eliminates motion of the joint, thereby reducing or eliminating pain at that level. Success rates for pain elimination are high for this method of treatment; however, since the entire spine works as a flexible load bearing system, fusion often results in other complications.
Fusing the spine at one or more levels alters the biomechanics of the spine at every other level above and below the fusion. If one level is fused, the loads are absorbed by one less disc into a system, which is not designed for such a change. Thus, the remaining discs must redistribute loads, each disc absorbing a greater load. In addition, the spine naturally flexes to absorb loads. A fusion alters the means by which the spine flexes. This also increases the loads on the remaining healthy discs. In turn, it is well understood that a complication of fusion is that additional fusions may be required in the future as other discs deteriorate due to the altered biomechanics of the spine. In other words, short-term pain relief is exchanged for long-term alterations to the spine, which, in turn, usually require further alterations by way of surgery.
There are numerous prior art patents addressing the issue of disc replacement. U.S. Pat. Nos. 6,443,987 B1 and 6,001,130, both to Bryan, disclose polymer composite structures for cushioning intervertebral loads. U.S. Pat. No. 5,258,031 to Salib, et al. and U.S. Pat. No. 5,314,477 to Marnay, disclose ball and socket-type implants addressing the issue of intervertebral mobility. These patents are exemplary of a first approach in the art using an elastomer as a motion and dampening structure and a second approach utilizing a ball and socket joint to create a moving pivot joint. There are many variations on these concepts, many include mechanical springs and are more complex structural mechanisms. A significant portion of the prior art addresses the issue of intervertebral motion, but do not address anatomical loading considerations, nor do the prior art address the possibility of multiple implants and the problems involved therewith. Problems such as stabilizing the intervertebral space are addressed, but other problems such as restoring anatomical motion to the vertebrae, especially where multiple implants are employed, are not addressed by the prior art.
The current state of the prior art artificial intervertebral discs are associated with various problems. For example, a number of implants constructed from polymers are of insufficient strength to work effectively in the higher loading areas, such as the lumbar area. Such polymers often take compressive sets so that the original height of the implant decreases over time. The surgeon must either compensate for the compression by initially using a larger polymer prosthesis and estimate compression, or use the approximately sized polymer prosthesis and later surgically replace the same once the irreversible compression of the prosthesis is unacceptable. This is commonly experienced where the implant is an attempt to mimic the disc structure and flexibility.
Implants constructed with ball and socket joints severely restrict or eliminate shock cushioning effectiveness of a normal disc. This type of implant can provide motion, but biomechanically, the ball and socket joint negatively affects other healthy discs of the spine. The result can be long-term problems at other levels of the spine, as seen with the current treatment of fusion.
Other implants, not discussed above, utilize bearing surfaces usually having polyethylene bearing against metal interfaces. Polyethylene is a bearing surface that is problematic in large joint replacement due to the wear properties of the material. Since artificial discs are intended to be implanted over long periods of time, such wear can be highly damaging to surrounding tissue and bone.
To insert an artificial disc assembly or like intervertebral implant, distraction of the otherwise collapsed intervertebral space is necessary. The intervertebral space is collapsed due to the deterioration of the natural disc, which has lost fluid therefrom, has lost structural integrity, or has a combination of both. Usually, an instrument is inserted into the intervertebral space and expanded, either by ratcheting, a screw mechanism, or the like to expand the intervertebral space wide enough to accept the spinal implant assembly. The intervertebral space is usually hyper-extended and then allowed to collapse about the implant. This hyperextension strains small muscles and otherwise strains the integrity of the joint structure.
In view of the above, it is desirable to provide a spinal implant assembly that stabilizes an intervertebral space and restores anatomical motion to the vertebrae defining intervertebral space. Further, it is desirable to provide the spinal implant assembly that does not require hyperdistraction of the intervertebral space in order to avoid trauma to the surrounding tissue.